To prevent a loco's battery slowly discharging when not in use, BPS includes an electronic switch to isolate the battery from all external components. I'll attempt to answer several questions related to how battery on/off is actuated.

Turn OnIf a loco has rail pick-up connections, BPS turns on whenever track voltage is detected. If the loco has no rail pick-up or track is not powered, a magnetic sensor mounted on the BPS circuit board is actuated with a magnetic wand. This requires the sensor to be mounted where it's convenient to bring the wand within actuation distance (about 1/2 inch). Where it's not convenient to mount the BPS close to roof or side of a loco, a sensor can be separated from the circuit board and glued directly to a suitable non-magnetic surface using appropriate adhesive or epoxy.

Turn OffBPS is designed to be turned off using a decoder function command, but differences in decoder behavior created so much confusion early in the life of BPS that a push-button is included as the default method of turn-off. The following discussion deals with turn-off procedures, but first, a few more remarks regarding the default push-button switch. Any momentary contact connecting (shorting) the orange colored wires is all that's required. A magnetic sensor (same component as used for turn-on) will work.Another point to remember: Any charging input (from track pick-up or a charging socket) overrides turn-off. Neither push-button nor decoder function command will turn off battery power when charging input is present.

Turn-off using a decoder function commandDecoder function outputs control devices (lights, for example) by "grounding" the appropriate output. For example, a loco's headlight is connected to a decoder's white and blue wires where blue is positive common and white is the function output intended for headlight control. A decoder turns headlight on by connecting its white lead (thru a transistor) to ground.

Looking at the BPS, one of the orange leads is a ground connection; the other (call it the shutdown lead) performs turn-off. If a decoder function output is connected to the shutdown lead, a function 'on' command will ground it and turn off the battery, assuming there is no input charging power. When the battery turns off, everything shuts down, including the decoder, which is exactly what we want.

So where's the confusion?When turned off, some decoders (NCE, for example) "remember" the state of their function outputs; others (SoundTraxx Tsunami, for example) forget function outputs and there is no confusion in this case. Shutdown using a Tsunami decoder is documented elsewhere.

Confusion occurs when attempting to turn on a BPS that was turned off by a function command from a decoder that remembers its state during shutdown. Suppose the loco has no track power and the magnetic wand is used to turn battery on. As soon as the decoder starts, it turns on its remembered function output, which turns off the battery and everything shuts down. A user perceives this behavior as a BPS problem.

What happens if the loco has track power? In this case, track power overrides the decoder's function output and turn-on appears normal. However, the decoder will turn off battery power as soon as charging input is removed. To the user, everything is normal until the loco moves onto unpowered track and stops. Again, it looks like a BPS problem.

A turn-on procedure that always worksThis procedure works because track power (charging input) overrides any attempt to turn off the battery. As soon as the decoder turns on, send a function command to turn off the remembered function output. That's it; no confusion, problem solved.However, the situation without track power requires holding the magnetic sensor on (using the magnet wand in one hand) while using the throttle (in the other hand) to send a function-off command. Definitely not convenient.

ExampleHere's an example based on a question from a modeler wanting to charge batteries while locos are idle in an engine shed or roundhouse where the building prevents manual access to turn battery on or off. A similar situation occurs with inaccessible storage sidings. For this example, the decoder is one that remembers its function outputs during shut down.Since the most convenient time to charge batteries is at the end of an operating session, the loco, having completed its tasks, enters the engine-shed where rails are powered and power is on for battery charging. A decoder battery-off command is then sent to the loco and is executed by the decoder, but charging input keeps battery power on. After several hours of charging, loco-shed rail power is turned off and the decoder, with function output on from its previous command, immediately turns off battery power. This procedure reduces risk of forgetting to turn off the battery and later discovering a flat battery.When commencing a new operating session, first turn on engine-shed track power, then transmit the appropriate battery-on (function-off) command and begin operation.

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I'm a retired electrical engineer, but still spending more time on engineering than on my layout. These days, it's mostly about applying radio control and battery power on smaller scale layouts (HO, On3, On30)